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The post pretty much says it all. I just acquired AtariGraphics and the accompanying light pen, but when the cartridge is inserted, the machine boots into the built in diagnostics as of option was held down. Other cartridges seem to work fine, and the machine goes to "READY" as expected with no cartridge inserted.

Is this a sign of a bad cartridge, or is this expected behavior under some circumstance?

If you haven't already tried this, give the contacts on the edge connector at the bottom of the cartridge a rub with an pencil eraser. I have had so many weird symptoms from dirty cartridge connectors.